The present invention relates to a lithium secondary battery which maintains a good charge-discharge cycle and in which safety may be secured with electricity being cut off when an excess current has occurred due to external short circuit, etc. so that the battery may not be exploded nor be ignited, and in particular, to a lithium secondary battery which may be preferably used for driving a motor of an electric vehicle, etc.
In recent years, in midst that it is eagerly desired to regulate the emission of exhaust gas including carbon dioxide and other harmful substances with the elevation of the environment protection campaign as a background, the campaign to promote an introduction of an electric vehicle (EV) and a hybrid electric vehicle (HEV) has become active in replacement of automobiles using fossil fuels such as a vehicle driven by gasoline in the automobile industry. A lithium secondary battery as a motor-driving battery acting as a key for putting such EV as well as HEV into practical use, is required to have not only huge battery capacity but also a huge battery output much affecting acceleration performance as well as gradeability of a vehicle, and on the other hand, however, a strict safe standard has been established from the point of view of securing safety since the battery is provided with high energy density.
In general, the internal electrode body of a lithium secondary battery comprises a positive electrode, a negative electrode and a separator made of porous polymer film, the positive electrode and the negative electrode being wound or laminated via the separator so that the positive electrode and negative electrode are not brought into direct contact with each other. For example, as shown in FIG. 1, an internal electrode body 1 of winding type is formed by winding a positive electrode 2 and a negative electrode 3, having a separator 4 in between, and tabs 5 are provided for each of positive and negative electrodes 2, 3 (hereafter referred to xe2x80x9celectrodes 2, 3xe2x80x9d) respectively. And, the ends opposite to the ends connected with electrodes 2, 3 of each tab 5 are attached to an external terminal (not shown) or an electric current extracting terminal (not shown) being conductive to the external terminal. That is, the tab 5 serves to act as a lead line (a current path) being conductive to the external terminal, etc. together with conducting current collecting from electrodes 2, 3.
Here, a plan view of the electrodes 2, 3 when the internal electrode body 1 is spread out is shown in FIG. 2. The electrodes 2, 3 are formed with an electrode active material being coated respectively onto metal foils 15 made of aluminum, etc. for the positive electrode 2 and made of copper for the negative electrode 3 respectively as current collecting bodies, thus forming an electrode active material layer 16.
The tab 5 is provided on one side of such a metal foil 15, and those having thin band shape are preferably used so that the portion where the tab 5 of the electrodes 2, 3 are attached may not swell to the direction of a periphery when the internal electrode body 1 was formed. In addition, they are preferably disposed in approximately uniform distance so that one tab 5 conducts current collecting from a constant area in the electrodes 2, 3. Incidentally, in general, a material to be used for the tab 5 is the same as a material of the metal foil 15 to which the tab 5 is attached.
Incidentally, with respect to a lithium secondary battery for an EV or an HEV, it is necessary to use lithium secondary batteries with a voltage of around 4 V at the highest for a single battery, such single batteries in plurality being connected in series, since a constant voltage is required to drive a motor, and, however, there is a case where discharge of a large current not less than 100 A is required to obtain the herein desired acceleration performance or gradeability. For example, maintaining that 200 V with 100 A be required and 3.6 V be an average terminal voltage at the time of discharge thereof, 56 units of single batteries are required to be connected in series, resulting in 100 A current flowing at each single battery at this time.
The internal configuration of a battery must be designed so that also in the case where such a huge current flows, the battery may normally operate while the output loss is suppressed as low as possible. Therefore, paying attention to the current path from the above described internal electrode body 1 and the external terminal, it is deemed preferable that the resistance of members themselves of the electrodes 2, 3 as well as the metal foils 15, or the tabs 5 and the external terminals, etc. all configuring the electrodes 2, 3 is small.
However, judging from the point of view of securing battery capacity as well as securing mechanical strength of electrodes, few degrees of freedom in setting quantity of the electrode active material layer 16 configuring the electrodes 2, 3 and sizes of the metal foils 15 are permitted while as concerns the electric current extracting terminal 13, normally considering the shape of batteries, or the energy density thereof, the quantity of the maximum discharge current, light-weight low-resistance members with resistance values not more than a predetermined value within a range which is possible to set are used.
On the other hand, the tab 5 has an allowable range to set up a resistance value on a point of view of feasibility to set up its shape freely as far as the shape of the tab 5 is to be housed in the space between the battery case housing the internal electrode body 1 therein and the internal electrode body 1. Metal members are used for the tab 5, whose resistance value is generally made smaller, nevertheless, the rate of the resistance value of the tab occupying the total internal resistance of a lithium secondary battery is not necessarily small, and cannot be ignored.
On condition that a plurality of above described tabs 5 in the shape of foil band are used, a tab 5 adopting a larger cross-sectional area to make the resistance value smaller will result in introducing a situation where energy density of a battery gets decreased since the total weight of the tabs 5 will become heavier in spite that effective decrease in the internal resistance and effective decrease in the output loss is expected.
On the contrary, making the cross-sectional area of the tabs 5 smaller decreases the total weight of the tabs 5 and increases the battery""s energy density, but on the other hand there will occur such problems that the resistance value of the tabs 5 increases, the tabs 5 the fuse due to increase in output loss because of increase of internal resistance or heat generated by current, and thus functioning as a battery will disappear. Accordingly, from the standpoint to avoid such problems and to do well both in reducing output loss and in increasing energy density, cross-sectional area of not less than a certain value is required for the tabs 5.
On the other hand, with respect to the above described problems, it is feared that an accident involving an explosion or an ignition may occur when a great current has been discharged at a time due to an external short circuit, etc. since a lithium secondary battery has higher energy density, and for the purpose of avoiding such situation in advance, xe2x80x9cGuideline for safety evaluation on secondary lithium cells (hereafter referred to SBA Guidelines)xe2x80x9d published by Battery Association of Japan regulates that a lithium secondary battery should be free of burst or ignition to be evidenced by an external short circuit test. To fulfill such a standard, in a lithium secondary battery various safety devices such as a current-limiting mechanism comprising a PTC element, a release mechanism of battery""s internal pressure involving safety valves, and pressure joints, etc. are incorporated or proposed.
Here, a current fuse is utilized in various electric appliances, but has never been used up to date as a current cutoff mechanism to be disposed inside a lithium secondary battery since a size or a shape of the current fuse is subject to a limit. However, if the tab 5 can function as the current fuse, with which an existing safety device may be replaced or concurrently disposed, it is deemed that a safety increase may be planned.
In the case where the tab 5 is used as a current fuse like this, the current cutoff value must be determined so that the tab 5 is fused with a predetermined quantity of excess current, but as mentioned above, there is naturally a limit in the structural shape of the tab 5. That is, for the purpose of using the tab 5 as a current fuse, the cross-sectional area of the tab 5 must be set not more than a predetermined value, but at the same time, considering the fact that the quantity of excess current may also defer due to the quantity of single battery""s internal resistance, it is regarded as necessary to set the cross-sectional area of the tab 5 in accordance with the quantity of a single battery""s internal resistance.
The present invention was attained by contemplating the problems of the prior art mentioned above, and its first purpose is to provide a lithium secondary battery having realized reduction in output loss and increase in energy density, and its second purpose is to provide a lithium secondary battery which has been planned to secure and increase in safety by incorporating tabs into the battery as a current fuse, being a replacement for a conventional safety device or to be concurrently disposed, and furthermore its third purpose is to provide a lithium secondary battery which concurrently realizes these characteristics, that is, reduction in output loss and increase in energy density, and security of safety having given tabs a function as a current fuse.
That is, according to the present invention, there is provided a lithium secondary battery, comprising:
an internal electrode body including a positive electrode, a negative electrode, and a separator, the positive electrode and the negative electrode being wound or laminated via the separator so that the positive electrode and the negative electrode are not brought into direct contact with each other;
an organic electrolyte; and
at least a plurality of tabs to be connected to each of the positive and negative electrodes for current collecting, the tabs having a total cross-sectional area of the tabs not less than a constant area in accordance with the quality of the material to be used for the tabs so that the tabs to be connected to each of the positive and negative electrodes may not fuse when at least 100 A current flows through the lithium secondary battery.
In such a lithium secondary battery of the present invention, the relationship between material of the tab and total cross-sectional area of the tabs is preferably not less than 0.009 cm2 for aluminum, not less than 0.005 cm2 for copper, and not less than 0.004 cm2 for nickel, further preferably not less than 0.014 cm2 for aluminum, not less than 0.008 cm2 for copper, and not less than 0.008 cm2 for nickel. A thickness of the tab is preferably not more than twice the thickness of an electrode active material layer in an electrode to which the tab is welded, and further preferably not more than the thickness of an electrode active material layer, that is, it is preferred that the thickness is set within a range where the portions where the tabs have been attached shall not swell when the tabs are attached to electrodes to be wound or laminated. Incidentally, from the point of view of reduction in internal resistance, a sum of resistance value of the tabs per a unit battery is preferably not more than 1 mxcexa9.
In addition, according to the present invention, there is further provided a lithium secondary battery, comprising:
an internal electrode body including a positive electrode, a negative electrode, and a separator, the positive electrode and the negative electrode being wound or laminated via a separator so that the positive electrode and the negative electrode are not brought into direct contact with each other;
an organic electrolyte; and
at least a plurality of tabs to be connected to each of the positive and negative electrodes for current collecting,
wherein the tabs function as current fuses.
In such a lithium secondary battery, the relationship between material of the tabs and the total cross-sectional area of the tubs when internal resistance of a unit battery is set R (mxcexa9) is preferably not more than 0.36/R (cm2) for aluminum, not more than 0.18/R (cm2) for copper, and not more than 0.14/R (cm2) for nickel, and more preferably not more than 0.18/R (cm2) for aluminum, not more than 0.09/R (cm2) for copper, and not more than 0.07/R (cm2) for nickel. In addition, if the tab is provided with a narrow portion, the tab is easily made to function as a current fuse, which is preferable.
It is preferable that the internal resistance in the above described lithium secondary battery of the present invention is not more than 10 mxcexa9 per a unit battery. In addition, by setting the relationship between material of the tab and total cross-sectional area of the tabs at not less than 0.008 cm2 and not more than 0.36/R cm2 for aluminum, not less than 0.005 cm2 and not more than 0.18/R cm2 for copper, and not less than 0.004 cm2 and not more than 0.14/R cm2 for nickel, and further preferably at not less than 0.014 cm2 and not more than 0.18/R cm2 for aluminum, not less than 0.008 cm2 and not more than 0.09/R cm2 for copper, and not less than 0.008 cm2 and not more than 0.07/R cm2 for nickel, a battery having the characteristics of the above described two kinds of lithium secondary batteries can be obtained.
Incidentally, when deviation of respective resistance values of tabs remains within xc2x120% of an average value, fusing at one tab causes increase in current flowing through the other tabs without making a large current flow into one tab with priority since difference in quantity of current related to the tabs is small, thus fusing of the tabs can be controlled not to occur in a chained fashion. It goes without saying that lack of variance in shape of respective tabs is preferred for the purpose that such deviation of resistance values of tabs is made smaller, and moreover, when an end part of tabs opposite to the end connected with electrodes is connected by pressure attachment, welding or eyelet, deviation of resistance for each tab having been connected with a battery can be reduced and preferable.
The characteristics of the above described lithium secondary battery of the present invention are preferably adopted as a lithium secondary battery with battery capacity of not less than 5 Ah, and the lithium secondary battery of the present invention is preferably used for an electric vehicle (EV) or for a hybrid electric vehicle (HEV).